1. Field of the Invention:
This invention relates to a method and an apparatus for forming a multicolor image by successively forming different color toner images on an image retainer in the fields of electrostatic recording and electrophotographic reproducing applications.
2. Description of the Prior Art:
Heretofore, multicolor images have been obtained through the electrophotographic reproducing method wherein a series of reproducing processes, namely, charging, exposing, developing and transferring, is repeated on a component color basis and a plurality of different color toner images are piled on and transferred to transfer paper. For instance, an electrostatic image is formed separately through the processes by blue, green and red obtained through a separation filter and developed using yellow, magenta, cyan and, if necessary, black toner to form toner images which are superposed to form a multicolor image. However, this method of forming a multicolored image has disadvantages including the necessity of transferring the image to a transfer substance each time development by colors is completed, larger equipment and lengthy image-forming time; (2) necessity of assuring that the several color images are not out of register, i.e., misaligned relative to the repetition of the reproducing operation.
Although there has been devised a method for forming a multicolor image while solving the problems above by piling up and developing a plurality of toner images on a photosensitive member and completing the transfer process at one time, that method still has disadvantages in that the toner image obtained in the preceding stage is disturbed or a multicolor image lacks color balance because the developer in the following stage is blended with the toner contained in the developer in the preceding stage.
In order to remedy these shortcomings, there has also been disclosed a method, for instance, Japanese Patent Laid-Open No. 56-1144452, comprising preventing a photosensitive member from contacting a developer layer for developing a latent image formed on the photosensitive member, and laying an a.c. component on a d.c. bias applied to a developing device to fly the toner contained in the developer across a gap. In this method of forming a multicolor image, the image is not disturbed because the developer layer is prevented from rubbing the toner image formed in and up to the preceding stage. Referring to a flowchart of FIG. 1, the principle of the image forming method will be described. FIG. 1 illustrates changes in potential on the surface of a photosensitive member positively charged. In FIG. 1, there are shown an exposed portion PH of a photosensitive member, a non-exposed portion DA of the photosensitive member, a rise DUP in the potential produced by the positively charged toner T stuck to the exposed portion PH in the first development, and a rise CUP in the potential produced thereby in the exposed portion PH due to the second development.
The photosensitive member is uniformly charged by a scorotron charge device or the like and provided with a constant surface potential E. The surface potential E on the exposed portion PH is reduced to almost zero by first image exposure by means of a light source such a laser, cathode raytube or LED. At this time, the d.c. component causes a positive bias roughly equivalent to the surface potential E in the non-exposed portion to be applied to a developing device and the positively charged toner T in the developing device is allowed to stick to the exposed portion PH having a relatively low potential, so that a first visible image may be formed. The potential in the region where the visible image has been formed is increased by the DUP because of the positively charged toner T adhering thereto and, as the region is charged secondly by the charge device, the potential is further raised by the CUP, whereby the initial surface potential E is obtained as in the case of the non-exposed portion. Subsequently, second image exposure is provided on the surface of the photosensitive member where uniform surface potential has been obtained to form an electrostatic latent image and a second visible image is obtained through the similar developing operation. A multicolor toner image is obtained on the photosensitive member by repeating the above processes and the image is transferred to recording paper and fixed with heat or under pressure to obtain a multicolor image. The toner and charge left on the photosensitive member is cleaned in preparation for the formation of the following multicolor image. In the above method of forming a multicolor image, the second and following charging may be omitted. In case the charging is repeated each time without the omission, a charge eliminating process may be added before the charging. Moreover, the exposure beam source used for each image exposure may be a similar or different one.
In the above method of forming a multicolor image, for instance, yellow, magenta, cyan and black color toner images are often superposed on the photosensitive member and the reason for this includes the following: Although a black image should be obtained by superposing the three primary colors of yellow, magenta and cyan according to the principle of the subtractive color process, clear black characters and diagrams can hardly be reproduced only by the three primary colors, because toner in actual use for the three primary colors has not an ideal adsorptive wavelength range and these color toner images are not easily positionally synchronized.
As a result, it is arranged to obtain a fourcolor image which is a more faithful reproduction of the document by superposing black as well as three primary color toner images as above described.
In the method of forming a multicolor image, reversal development is also used for developing an electrostatic latent image. In reversal development, it is only necessary to expose a portion where a toner image is formed on the photosensitive member but not to expose the background without any gap as is the case with normal development, so that a latent image may relatively readily be formed on the photosensitive member with a toner image already formed. Moreover, the advantage is that the life of the photosensitive member can be prolonged as it is wear-resistant. Further, because the second and following charging are effected at the same polarity as the toner, electrostatic transfer is implemented without trouble.
As a method of forming a latent image for forming a multicolor image, there are those of forming a latent image by directly injecting a charge into an image retainer using a multi-stylus electrode and of forming a magnetic latent image using magnetic head in addition to that of forming an electrostatic latent image by uniformly charging the photosensitive member and image exposure.
Although each of these methods of forming a latent image allows the expression of gradation, the problem is that they are not suitable for high-speed recording. Moreover, because the gradation thereby expressed through such methods is the so-called multistage gradation, a greater capacity for image data is required. Accordingly, there has been proposed a method for providing image data of gradation in the form of binary values, the method comprising converting each picture element to a binary value for recording purposes and expressing dummy gradation based on the distribution of the binary values to minimize the capacity of image data. The density pattern method of FIG. 2 and the dither method of FIG. 3, for instance, are used to express the gradation of an image through the above method of forming image data of gradation in the binary form.
The density pattern method shown in FIG. 2 presupposes the conversion of one picture element into a plurality of elements. In FIG. 2, there are shown a document 1a, each picture element 5a having gradation; a sample 2a for extracting a picture element 5a representing the typical density of a matrix of the document 1a and processing the value in terms of a threshold; a matrix 3a having the threshold density of MxN corresponding to the sample; and a pattern 4a provided in a binary form by comparing the threshold matrix 3a and the sample 2a.
The dither method shown in FIG. 3(a) is intended to convert a picture element into i picture elements A document 1b is divided into density matrices on a MxN picture element basis. A sample 2b is subjected to a threshold process corresponding to the density matrix of the document 1b, the threshold density matrix 3b of MxN corresponds to the sample 2b, and a pattern 4b is represented by a binary value obtained by comparing the threshold matrix 3b with the sample 2b.
In the conventional method of expressing gradation, it has been preferred to arrange dots in such a manner as to set a space frequency greater. In other words, the gradation is, as shown in FIG. 2 or 3(a), expressed by the number of dots of predetermined size (dot density). Particularly in the dither method, deterioration in resolution has been considered minimizable. However, in the aforementioned method of forming a multicolor image, gradation is incapable of being satisfactorily expressed because the resolving power is reduced as dots are welded together or an image looks coarse when the image is formed through the developing, transferring and fixing processes. The problem is that, for instance, even the resolving power in the order of 16 dots/mm required for the formation of an ordinary image cannot be maintained.
There are two methods of expressing the gradation of a multicolor image: (1) different color dots are prevented from overlapping; (2) different color dots are allowed to overlap at least partially. In the case of (1), the dots are formed in different places within the pattern 4a or 4b as shown in FIGS. 2 and 3(a). Accordingly, different color dots are distributed separately and two-dimensionally and a dummy mixture of colors is formed on recording paper.
In the case of (2), because different color dots are allowed to exist together within the pattern 4a or 4b, the different color dots are at least partially overlapped. In the case of (2), though development is implemented while the latent potential and the development bias are controlled, the formation of a desired latent image is not achieved because the overlapped potential is short and the toner dot which has already been developed impairs image exposure for the formation of the following toner dot. As a result, the tone of the preceding toner dot is excessively emphasized, which poses a problem in that the color balance of a multicolor image is broken. This constitutes a serious problem when the picture element is converted into a binary value to express the color balance. Particularly when the method of expressing decentralized gradation shown in FIGS. 2 and 3(a) is used, the problem becomes still more serious.
Even when the different color dots are not allowed to overlap in the case of (1), the same type of problem occurs because of an unavoidable error in positioning which is caused when a latent image of the different color toner image is formed and because of the diffusion of the dots. The problem is conspicuous when the method of expressing the decentralized gradation as in the case of (2) is employed.
When the aforementioned reversal development is used to form a color toner image on a photosensitive member, the following problems are posed: That is, as light for exposing an image is barely transmissible through a region where the toner has adhered from the development in the preceding stage, and the surface potential is thus sufficiently lowered, the toner is not allowed to stick to the photosensitive member in the following development stage. Even in the case of additive processes, because of difficulties in complete positioning and complete development corresponding to an electrostatically charged image, the same problem occurs. Accordingly, even if it is attempted to develop three primary yellow, magenta and cyan colors successively to express various tones, there will also be posed problems including the disturbance of the color balance and the image near the peripheral edge. Thus, a desired color image is not formed.
Heretofore, a bulb, fluorescent lamp, EL (electroluminescence) or LED (light emitting diode) has been used as a light source for providing image exposure on a photosensitive member in an apparatus for forming an image but the use of a laser as a light source for image exposure is on the increase. In other words, the laser beam offers special properties such as greater energy per area unit, coherence and higher directivity and, because it permits the formation of an image of good quality at high speed without noise, much importance has been attached thereto.
In the apparatus for forming an image using the laser beam, there is used, for instance, a He-Ne or He-Cd laser capable of emitting beams whose wavelength band ranging from 400 to 600 nm equivalent to the beam absorptive wavelength of a photosensitive member for forming an ordinary image.
A laser beam L.sub.3 for image exposure is generated by a laser-beam exposure device 1 shown in FIG. 3(b). In FIG. 3(b), a signal 7 from a signal source 6 based on, for instance, image data, a facsimile or computer is applied to a driver 8 and an optical mudulator 5 such as an EOM {Electric Optical Modulator) or AOM (Acoustic Optical Modulator) is driven by the driver 8 and the intensity of a laser beam L.sub.1 from a light source 2 is modulated. A laser beam L.sub.2 after modulation is reflected from the reflecting surface of a polygon 9 rotate at high velocity and the laser beam L.sub.3 thus reflected is irradiated on a photosensitive member 112 to form an electrostatically charged image. A lens 3 is one capable of converging at a diameter (for instance, 50 to 300 .mu.m) where the laser beam L.sub.1 can be modulated by the modulator 5, whereas a lens 4 is a collimate lens for obtaining the parallel laser beam L.sub.2 after modulation. A lens 10 is a focusing f.theta. lens and 111 shows a scanning area by the laser beam L.sub.3.
A gas laser is used as a beam source for image exposure in an apparatus for forming a multicolor image and, as a gas such as helium or neon is used as a substance for a conventional laser beam source, the disadvantages pointed out include an increase in the size of the beam source means and the price thereof. As shown in FIG. 3(b), the driver 8, the beam modulator 5 and the like are required to modulate the intensity of the laser beam L.sub.1 and the laser beam L.sub.3 must be generated without interruption while an image is being formed and this makes the amount of the light source energy enormous. Particularly in the case of the formation of a multicolor image, the amount of image data is large and therefore the exposure beam source should be the one which can be operated at high velocity and provide excellent tone reproducibility in order to maintain the color balance. Accordingly, a laser beam source in demand for an apparatus for forming a multicolor image should be not only capable of modulating the intensity of light in proportion to image data but also compact and less costly.